Insulin-like growth factor binding proteins (IGFBPs) are expressed in lung from early in gestation and may modulate IGF-stimulated fetal lung cell proliferation and/or differentiation. To begin to define IGFBP production and regulation in lung cells during development, we prepared primary cultures of 19 day gestation fetal rat lung fibroblasts and epithelial cells and identified IGFBPs secreted into medium. Ligand blot analysis of conditioned media (CM) from both cell types demonstrated IGFBP bands of approximately 39,000-45,000, 32,000, 24,000, and 22,000 M(r). These migration characteristics allowed the identification of the 39,000-45,000 M(r) bands as IGFBP-3 and the 24,000 M(r) band as IGFBP-4, while Western immunoblot analyses localized IGFBP-2 to the 32,000 M(r) band and IGFBP-5 to the 22,000 M(r) band. Polymerase chain reaction amplification of cDNAs generated by reverse transcription of fibroblast and epithelial cell RNA using specific oligodeoxynucleotide primers for IGFBPs 1 through 6, demonstrated the presence of amplified products for IGFBP-2, -3, -4, -5, and -6. In both cell types, IGFBP-2 and -3 production was sustained during 48 h of incubation in serum-free medium, whereas IGFBP-4 abundance increased only during the first 6 to 12 h of incubation. CM from fibroblasts and epithelial cells plated at low densities contained a high abundance of IGFBP-2 per microgram cellular DNA compared with cells at higher densities. In contrast, IGFBP-3 and -4 abundance normalized to cell DNA did not change with differing cell densities.(ABSTRACT TRUNCATED AT 250 WORDS)